Anytime you are looking at an image on the monitor at something other than 100% there is an amount of scaling being done by the software to make it fit on the monitor. Sometimes (very important word), that scaling will cause jaggies. Simply viewing the image slightly larger or smaller will completely change the scaling ratio and eliminate the artifacts.

I don't know that this is the case with your image. If viewing at different sizes always shows jaggies, it is obviously an issue with the image data itself. JPEG compression is always a likely suspect when dealing with JPEG files.